Problem with AMD Prorender install on MAC Catalina Blender 2.81
The problem could be that there is an extra file you need to download to get the add-on to work. I know for a fact that for LuxCore render there is a similar issue, and that was solved by another file that needed to be downloaded for it to work. Another trick that might work in case there is no other file that needs to be installed would be to install the add-on in Blender manually. You would need to do this by looking at the start up file location for Blender, and searching until you find the file for add-ons. (This can be easily done by looking at the file location info Blender gives you in the info section of an add-on.)